This invention relates to a novel method for casting a base directly on an electron tube and particularly to producing the base by injection molding.
Many types of electron tubes, such as television picture tubes, comprise an evacuated envelope including a stem and an array of relatively-stiff stem leads sealed in, and extending out from, the stem. Typically, the stem is a circular glass disc having a central opening from which the exhaust tubulation extends and a circular array of stem leads around the tubulation.
It is common practice to attach a prefabricated base, usually of some type of plastic material, over the stem with the stem leads extending therethrough where it is held by friction or with an adhesive. Where one or more leads are to carry relatively high voltages, the lead may be surrounded by a dielectric material to suppress interelectrode arcing. The prefabricated bases are usually assembled to the stems manually, a process that often leads to high cost and great variation in quality.
It has been suggested in U.S. Pat. No. 2,433,373 issued Dec. 30, 1947 to N. B. Krim to cast the base directly to the stem by injection molding. That prior method provides a mold with a temporary pressure seal radially around the tube above the glass stem in order to prevent leakage from the pressure of the injection molding material. Soft leads that extend from the stem attach to stiff leads that are mounted in an auxiliary plate. The stiff leads are inserted in bores in the mold during molding. With this arrangement, little or no stress is applied to the stem through the leads when the pressure seal is applied. However, this prior method is relatively slow, is laborious, uses excessive casting material and requires a special arrangement of base leads. The novel method is faster, can be practiced with an automatic or semi-automatic machine, uses substantially less casting material, and can be practiced on electron tubes having stiff stem leads extending from the stem of the tube. The novel method also overcomes the problem of producing a pressure seal over protuberances which may have been produced when the neck-to-stem seal was formed.